1. Field of the Invention
The present invention relates to a CO sensor and a method of measuring CO concentration, and more particularly to a CO sensor and a method adapted for measuring CO concentration of fuel gas in a fuel cell.
2. Description of the Related Art
In view of worldwide environmental considerations, in recent years active studies have been conducted on fuel cells which serve as a highly-efficient, clean power source. Among them, solid polymer electrolyte fuel cells (PEFCs) are expected to find use as a fuel cell for automobiles, because of their low-temperature operation and high output density. In this case, a reformed gas such as methanol shows promise as a fuel gas. Since CO is generated during a reformation reaction process, depending on conditions such as temperature and pressure, a resultant reformed gas contains CO. Since CO poisons fuel-electrode catalysts, such as Pt of a fuel cell, a CO sensor is needed which can directly detect CO concentration in the reformed gas. Further, such sensor must be able to measure CO concentration in a hydrogen-rich atmosphere.
In view of the foregoing, Japanese Patent Application Laid-Open (kokai) No. 8-327590, proposes a carbon-monoxide detection apparatus (CO sensor) capable of effecting measurement in a hydrogen-rich gas. The proposed carbon-monoxide detection apparatus includes an electrolytic film and two electrodes sandwiching the electrolytic film. A gas to be measured (hereinafter referred to as a xe2x80x9cgas under measurementxe2x80x9d) contacts one electrode; atmospheric air contacts the other electrode; and the CO concentration of the gas is obtained from a potential difference between the two electrodes, to which a predetermined load is connected.
3. Problems Solved by the Invention
In the apparatus disclosed in Japanese Patent Application Laid-Open No. 8-327590, CO concentration is obtained from a difference in potential between two electrodes sandwiching a proton-conductive electrolytic film. However, theoretically, the potentials vary with the hydrogen concentration of a gas under measurement. Therefore, variations in the hydrogen concentration make accurate measurement of CO concentration difficult.
It is therefore an object of the present invention to provide a CO sensor and a method of measuring CO concentration which enables accurate measurement of CO concentration irrespective of variations in hydrogen concentration of a gas under measurement.
The above-described problem of the prior art have been solved by providing a CO sensor comprising a first measurement space in communication with a measurement gas atmosphere via a first diffusion-controlling section, a second measurement space in communication with the first measurement space via a second diffusion-controlling section, a first proton-conductive layer, a second proton-conductive layer, a first electrode disposed in contact with the first proton-conductive layer and located within the first measurement space, a second electrode disposed in contact with the first proton-conductive layer and located outside the first measurement space, a third electrode disposed in contact with the second proton-conductive layer and located within the second measurement space, a fourth electrode disposed in contact with the second proton-conductive layer and located outside the second measurement space, and a support for supporting the first diffusion-controlling section, the first measurement space, the second diffusion-controlling section, the second measurement space, the first proton-conductive layer, the second proton-conductive layer, the first electrode, the second electrode, the third electrode, and the fourth electrode.
A gas under measurement is introduced into the first measurement space via the first diffusion-controlling section. By applying a first predetermined voltage between the first electrode and the second electrode, hydrogen within the first measurement space dissociates, decomposes, or reacts with another element to thereby generate protons. The thus-generated protons are transported between the first electrode and the second electrode via the first proton-conductive layer, so that the hydrogen concentration within the first measurement space is controlled to a constant level. CO concentration of the gas under measurement is obtained based on an electrical signal which is generated between the third and fourth electrodes upon introduction of the gas under measurement having a controlled hydrogen concentration from the first measurement space to the second measurement space via the second diffusion-controlling section.
In the above-described CO sensor, the electrical signal may be current which flows between the third and fourth electrodes upon application of a second predetermined voltage to the third and fourth electrodes, or electromotive force generated between the third and fourth electrodes.
In one aspect, the present invention is characterized in that a gas is prepared which is the gas under measurement containing CO and adjusted to have a constant hydrogen concentration, hydrogen gas generated by reaction of CO contained in the gas with a hydrogen-containing substance is decomposed or dissociated to thereby generate protons; and the CO concentration of the gas under measurement is obtained based on a limiting proton current which flows by transporting the generated protons through a proton-conductive layer.
The present invention also provides a CO-concentration measurement method which comprises introducing a gas under measurement into a first measurement space via a first diffusion-controlling section, and controlling hydrogen concentration in the first measurement chamber to a constant level by pumping hydrogen contained in the gas under measurement outside the first measurement space or pumping hydrogen into the first measurement space; introducing into a second diffusion-controlling section the gas under measurement present in the first measurement space and having a controlled hydrogen concentration, and reacting CO contained in the gas under measurement with a hydrogen-containing substance at the second diffusion-controlling section to thereby generate hydrogen; introducing into a second measurement space the gas under measurement present in the second diffusion-controlling section and containing the generated hydrogen; and obtaining a CO concentration of the gas under measurement based on a concentration or amount of hydrogen in the second measurement space.
The present invention also provides a CO-concentration measurement method which comprises introducing a gas under measurement into a first measurement space via a first diffusion-controlling section, and controlling hydrogen concentration in the first measurement chamber to a constant level by pumping hydrogen contained in the gas under measurement outside the first measurement space or pumping hydrogen into the first measurement space; introducing into a second measurement space, via a second diffusion-controlling section, the gas under measurement present in the first measurement space and having a controlled hydrogen concentration; reacting CO contained in the gas under measurement with a hydrogen-containing substance at the second measurement space to thereby generate hydrogen; and obtaining a CO concentration of the gas under measurement based on a concentration or amount of hydrogen in the second measurement space.
Next, the measurement principle of the CO sensor and the CO-concentration measurement method according to the present invention will be described.
(1) A gas under measurement is introduced into the first measurement space via the first diffusion-controlling section.
(2) By applying a first predetermined voltage between the first electrode and the second electrode, hydrogen contained in the gas under measurement introduced into the first measurement space dissociates, decomposes, or reacts with another element, whereby hydrogen is pumped outside of the first measurement space in the form of protons. The first predetermined voltage is a voltage which causes dissociation, decomposition, or reaction of hydrogen contained in the gas under measurement and which transports the protons thus generated through the first proton-conductive layer; preferably, a voltage which causes a limiting proton current flow.
(3) The gas under measurement having a controlled hydrogen concentration is introduced into the second diffusion-controlling section.
(4) In the second diffusion-controlling section, CO contained in the gas under measurement having a controlled hydrogen concentration reacts with H2O as shown in the following formula (1) to thereby generate hydrogen. The reaction of formula 1 may be induced at a location other than the second diffusion-controlling section, e.g., at the second measurement space.
CO+H2Oxe2x86x92H2+CO2xe2x80x83xe2x80x83formula (1) 
(5) The gas under measurement which contains hydrogen generated by the reaction of formula (1) is introduced into the second measurement space.
(6) CO concentration is obtained by measuring the concentration or amount of hydrogen contained in the gas under measurement introduced into the second measurement space. In addition to hydrogen generated by reaction of CO, the measured hydrogen may contain hydrogen which is contained, in a controlled amount or at a controlled concentration, in the gas under measurement introduced from the first measurement space to the second diffusion-controlling section.
First Method of Measuring Concentration or Amount of Hydrogen in the Second Measurement Space
By applying a second predetermined voltage between the third and fourth electrodes, hydrogen contained in the gas under measurement introduced into the second measurement space dissociates, decomposes, or reacts with another element, whereby hydrogen is pumped outside of the second measurement space in the form of protons. The second predetermined voltage is a voltage for dissociating, decomposing, or reacting hydrogen contained in the gas under measurement introduced into the second measurement space and for establishing a limiting proton current flow.
At this time, current flows between the third and fourth electrodes in proportion to the amount of hydrogen generated by reaction of CO; i.e., the CO concentration of the gas under measurement. Therefore, the CO concentration of the gas under measurement can be measured based on the current flow.
Second Method of Measuring Concentration or Amount of Hydrogen in the Second Measurement Space
Electromotive force is generated between the third and fourth electrodes in accordance with the difference between the concentration of hydrogen contained in the gas under measurement introduced into the second measurement space (hydrogen concentration at the third electrode) and the hydrogen concentration at the fourth electrode.
When the hydrogen concentration at the fourth electrode is constant, the electromotive force changes in accordance with the amount of hydrogen generated by reaction of CO; i.e., the CO concentration of the gas under measurement. Therefore, the CO concentration of the gas under measurement can be measured on the basis of the electromotive force.
According to the CO sensor and the CO-concentration measurement method of the present invention, the concentration of hydrogen contained in the gas under measurement introduced into the second measurement space is controlled by pumping out hydrogen contained in the gas under measurement at the first measurement space. Therefore, the current flowing between the third and fourth electrodes or the electromotive force generated between the third and fourth electrodes does not depend on the hydrogen concentration of the gas under measurement and depends solely on the CO concentration. Accordingly, the CO sensor according to the present invention enables measurement of CO concentration without being affected by the hydrogen concentration of the gas under measurement.